Chimpanzees and bonobos reinstate an interrupted triadic game
When humans engage in joint action, they seem to so with an underlying sense of joint commitment, a feeling of mutual obligation towards their partner and a shared goal. Whether our closest living relatives, bonobos and chimpanzees, experience and understand joint commitment in the same way is subject to debate. Crucial evidence concerns how participants respond to interruptions of joint actions, particularly if they protest or attempt to reengage their reluctant or distracted partners. During dyadic interactions, bonobos and chimpanzees appear to have some sense of joint commitment, according to recent studies. Yet, data are inconsistent for triadic games with objects. We addressed this issue by engaging N=23 apes (5 adult chimpanzees, 5 infant bonobos, 13 adult bonobos) in a "tug-of-war" game with a human experimenter who abruptly stopped playing. Adult apes readily attempted to reengage the experimenter (>60% of subjects on first trial), with no group differences in the way of reengagement. Infant bonobos rarely reengaged and never did so on their first trial. Importantly, when infants reengaged passive partners, they mostly deployed (tactile) signals, yet rarely game-related behaviours (GRBs) as commonly observed in adults. These findings might explain negative results of earlier research. Bonobos and chimpanzees may thus have motivational foundations for joint commitment, although this capacity might develop over lifetime. We discuss this finding in relation to evolutionary and developmental theories on joint commitment.